When I was learning to play the guitar as a boy back in the 1960s, my friends would get a blank look on their faces when I was playing my guitar, and then suddenly I would grab the phone (land line of course) and hold the receiver up to my ear. Little did they know, I was tuning my guitar. I was listening to the dial tone, which was actually two tones: An F and an A. 

As I listened, I focused my hearing on the higher of the two notes which was the A note, which I would use as a reference for tuning the A string on the guitar. I'd pluck the A note (the 5th string open on the guitar) and match the sound to the A note on the phone. Although the A notes on the guitar and the phone were in different octaves, they were both still A notes and could be matched. Try it sometime if you have access to a land line phone!